The present invention relates to water-soluble soldering flux and paste solder using the flux, particularly to water-soluble soldering flux and paste solder for use in soldering electronic parts to a printed circuit board.
Soldering flux which is used for soldering electronic parts usually comprises rosin as the main component. Such a rosin-containing flux has good solderability, and the flux residue after soldering is electrically insulating. It is also non-corrosive and non-toxic. However, sometimes it is necessary to wash-off the flux residues after soldering, since rosin is sticky and attracts dust and foreign particles, resulting in a degradation in the electrical insulating properties, especially when the particles absorb moisture. Therefore, flux residues are usually washed off with an organic solvent such as "Chlorothene (tradename)" and "Freon (tradename)".
Paste solder is also used for soldering electronic parts to a printed circuit board. The paste solder is prepared by combining solder powder with a flux composition. A suitable amount of paste solder is first applied to given areas on the board by means of screen printing or dispensers. Electronic parts are mounted on the paste solder and the printed circuit board is then heated to reflow the solder by means of a reflow furnace, a hot plate, a blow of hot gas, a laser hot steam, and the like.
In general, the following properties are required of paste solder:
(1) Suitable viscosity and good printability;
(2) Good solderability;
(3) Non-toxicity and odorlessness; and
(4) Constant viscosity and non-skinning even after a long period of storage.
Recently, as electronic equipment becomes smaller in size and higher in packaging density, land patterns on printed circuit boards are becoming finer and the distance between the neighboring lands are becoming smaller. Therefore, paste solder which is used for electronic equipment must have improved printability, ability of being washed off, and reliability.
Soldering flux and paste solder are classified into an oil-soluble type in which rosin is used as a base fluxing material, and a water-soluble type in which a water-soluble composition such as glycerin, polyethylene glycol, polypropylene glycol, or diethylene glycol is used as a base material.
After soldering, flux residues must be removed, and water and an organic solvent such as Chlorothene (tradename) and Freon (tradename) are used for the water-soluble type and the oil-soluble type, respectively.
Today however, strict environmental and health regulations restrict the use of organic solvents. Therefore, it is highly desirable to be able to use water as a cleaning agent instead of organic solvents.
Water has the following advantages as a cleaning agent:
(i) Water is non-combustible and is very easy to handle.
(ii) Water causes no environmental disruption.
(iii) Water is cheaper than any organic solvents.
(iv) Water is the best solvent for ionic substances such as the activators used in soldering fluxes.
Several types of water-soluble soldering fluxes are known including inorganic compounds such as zinc chloride and ammonium chloride dissolved in glycerin or vaseline and organic compounds such as formic acid and ammonium formate in an aqueous solvent. However, a water-soluble flux which contains organic salts is so active that corrosion easily occurs even if a very small amount of residues remains. Therefore, it is necessary to wash the flux extremely thoroughly. Fluxes which employ glycerin and vaseline as solvents are difficult to wash off with water when they are heated to high temperatures during soldering.
Furthermore, in case of paste solder, most of requirements (1)-(4) mentioned above can be satisfied by commercially-available oil-soluble paste solders. In contrast, water-soluble paste solders do not have satisfactory paintability and are difficult to wash off. These problems are mainly caused by a base carrier. Namely, oil-soluble paste solders use rosin as a base carrier, but there has not yet been found a base carrier for water-soluble paste solders which has the satisfactory properties of oil-soluble paste solders using rosin.
Water-soluble paste solders now available use polyethylene glycol and polypropylene qlycol as a base carrier. However, such water-soluble paste solders do not roll but instead slide laterally when moved on a screen with a squeegee, and the solder does not thoroughly fill the openings of the screen, resulting in a degradation in printability. This is because the paste does not have satisfactory viscosity and tack.